I have often wondered whether lichens might be studied to give some indication of a stones origins. Lichenometry is a geomorphic method of geochronologic aging that uses lichen growth to determine the age of exposed rock: lichens are presumed to increase in size radially at specific rates as they grow. Measuring the diameter of the largest lichen of a species on a rock surface can therefore be used to determine the amount of time that the rock has been exposed. Lichen can be preserved on old rock faces for up to 10,000 years, providing the maximum age limit of the technique, though is most accurate (within 10% error) when applied to surfaces that have been exposed for less than 1000 years. The use of lichenometry is of increased value for dating deposited surfaces over the past 500 years as radiocarbon dating techniques are less efficient over this period (Wikipedia 21.01.2012).
These images were taken in February 2011 at the boulder fields, Garrow.